Field of the Invention
The present invention relates to a rubber composition for vibration proof rubbers, and particularly to a rubber composition for vibration proof rubbers that is usable suitably for vibration proof members such as an engine mount for automobiles, and a vibration proof rubber using this composition.
Description of the Related Art
As a vibration proof rubber, a composition has been hitherto used in which carbon black as a reinforcing material is blended into natural rubber. In recent years, the market has been requiring vibration proof rubbers, particularly, vibration proof rubbers for automobiles to be decreased in dynamic magnification (“dynamic spring constant”/“static spring constant” ratio).
In order to make vibration proof rubbers lower in dynamic magnification, it is important to heighten the dispersibility of carbon black in their natural rubber. Hitherto, a method has been adopted in which as the species of carbon black, a species large in particle diameter is used to heighten the dispersibility of carbon black in natural rubber. However, this method tends to damage the endurance of vibration proof rubbers.
Apart from the above, in a rubber composition, a vulcanization promoter is generally used together with a sulfur-containing vulcanizer to shorten the vulcanizing period of the composition, lower the vulcanizing temperature thereof and decrease the amount of the vulcanizer. A substance for activating this vulcanization promoter to heighten a vulcanization-promoting effect thereof further is a metal oxide, atypical example thereof being zinc oxide (zinc flower). Patent Document 1 listed below states that a vibration proof rubber decreased in dynamic magnification can be obtained by vulcanizing a rubber composition in which zinc flower made into fine particles is blended into natural rubber in a specific amount for 100 parts by weight of the natural rubber. However, the inventors have made eager investigations to find out that in the technique described in this patent document, there remains a room for a further improvement from the viewpoint of a decrease of the vibration proof rubber in dynamic magnification.
Vibration proof rubbers, particularly, for automobiles may be intensely required to be decreased in dynamic magnification and be further improved in heat resistance. Patent Document 2 listed below discloses a technique of blending 3 to 50 parts by weight of a multiple zinc flower into 100 parts by weight of a diene based rubber in a rubber composition to decrease the resultant vibration proof rubber in dynamic magnification. However, the inventors have made eager investigations to find out that there remains a room for a further improvement from the viewpoint of an improvement of the vibration proof rubber in heat resistance.
Patent Document 3 listed below describes a technique of blending a mixed resin into a rubber composition to aim to provide a rubber composition for vibration proof rubbers that is able to restrain the vibration proof rubbers from being changed, with time, in dynamic spring constant in a region of very low temperatures. However, the invention described in this document is an invention for solving a problem in an especial situation such as a very low temperature situation. Thus, the document neither discloses nor suggests that the invention aims to decrease the resultant vibration proof rubber in dynamic magnification.